Typically, an adjustable chuck of a type in widespread use for gripping workpieces of different sizes comprises a trio of jaws, which are mounted for relative movement between an opened condition and a closed condition, and a screw-driven mechanism, which is operable selectively for closing the jaws to grip a workpiece and for opening the jaws to free the workpiece.
Commonly, the screw-driven mechanism is actuated by a hydraulic motor, via couplers enabling the chuck to be selectively coupled to such mechanism and uncoupled therefrom. Commonly, a plural number of such chucks are associated with a greater or lesser number of machine tools capable of employing such chucks interchangeably, in a manufacturing cell.
Adjustable chucks of the type noted above are exemplified in prior patents including Hohwart et al. U.S. Pat. No. 3,423,098 and Hohwart U.S. Pat. No. 3,472,526. Similar chucks, in which the jaws may be selectively arranged to grip externally or internally, are available commercially from ITW Woodworth (a division of Illinois Tool Works Inc.) of Detroit, Mich., under its trade designation "Universal Ball-Lok Power Chucks."
When such a chuck is used to grip a fragile workpiece, such as a thin-walled tube being gripped externally or internally, it is important to grip the workpiece firmly but not to distort the workpiece by causing the workpiece to become out-of-round or otherwise. It is desirable, therefore, to control the gripping force applied by the jaws to the workpiece within close tolerances. It is difficult to maintain close tolerances, however, because of such factors as manufacturing tolerances, surface finishes, and lubrication.
For many years, there has been a need, which heretofore has not been effectively addressed, for a better way to control the gripping force applied to a workpiece by the jaws of such a chuck. The need has been critical in some industries, such as the aircraft industry and the construction equipment industry.